These chemicals, some of which have been banned since the 1970s, have been linked to serious health conditions including cancer and problems with brain development.

PBDEs have been used in flame retardants but are now banned in many US states, as are organochlorine pesticides.

The research was carried out by the University of California in San Francisco (UCSF). Of the chemicals checked for, 99 to 100% of the women were found to have levels of the contaminants in their systems.

The concern was the exposure to multiple chemicals and what effect this may have on unborn foetuses.

Lead author and director of the UCSF Programme on Reproductive Health and the Environment, Tracey Woodruff said: "It was surprising and concerning to find so many chemicals in pregnant women without fully knowing the implications for pregnancy.

"Several of these chemicals in pregnant women were at the same concentrations that have been associated with negative effects in children from other studies.

"In addition, exposure to multiple chemicals that can increase the risk of the same adverse health outcome can have a greater impact than exposure to just one chemical."

The researchers say further research is needed to identify the dominant sources of exposure to chemicals and what influence they have on health. Alison Brown